Editing Systems Network Architecture (section)

===Logical Unit (LU)===
SNA essentially offers transparent communication: equipment specifics that do not impose any constraints onto LU-LU communication. But eventually it serves a purpose to make a distinction between LU types, as the application must take the functionality of the terminal equipment into account (e.g. screen sizes and layout).

Within SNA there are three types of data stream to connect local display terminals and printers; there is SNA Character String (SCS), used for LU1 terminals and for logging on to an SNA network with Unformatted System Services (USS), there is the [[IBM 3270|3270]] [[IBM 3270#Data stream|data stream]] mainly used by mainframes such as the [[System/370]] and successors, including the [[zSeries]] family, and the [[IBM 5250|5250]] data stream mainly used by minicomputers/servers such as the [[System/34]], [[System/36]], [[System/38]], and [[AS/400]] and its successors, including System i and [[IBM Power Systems]] running [[IBM i]].

SNA defines several kinds of devices, called Logical Unit types:<ref>{{harv|Schatt|1991|p=229}}.</ref>
*LU0 provides for undefined devices, or build your own protocol. This is also used for non-SNA 3270 devices supported by [[Telecommunications Access Method|TCAM]] or VTAM.
*LU1 devices are printers or combinations of keyboards and printers.
*LU2 devices are IBM 3270 display terminals.
*LU3 devices are printers using 3270 protocols.
*LU4 devices are batch terminals.
*LU5 has never been defined.
*LU6 provides for protocols between two applications.
*LU7 provides for sessions with IBM 5250 terminals.
The primary ones in use are LU1, LU2, and [[LU6.2]] (an advanced protocol for application to application conversations).